Carbon monoxide usually is obtained by separation from synthesis gases produced by catalytic conversion or partial oxidation of natural gas, oils or other hydrocarbon feedstock. In addition to carbon monoxide, these gases contain primarily hydrogen and methane but are often contaminated with significant amounts of nitrogen (derived from the feed or added during processing). Conventional cryogenic separation processing leaves nitrogen as an impurity in the carbon monoxide, which, for both environmental and processing reasons, is unacceptable for some uses of carbon monoxide. The problem of nitrogen contamination of carbon monoxide product is becoming an increasing problem with the usage of more marginal feed stock in front end reforming processes. Accordingly, there is a demand for efficient and effective removal of contaminant nitrogen from carbon monoxide product.
It has been proposed in U.S. Pat. No. 4,478,621 to overcome the problem by distilling nitrogen-contaminated carbon monoxide in a distillation column using nitrogen-freed carbon monoxide as a heat pump stream. In particular, nitrogen-freed carbon monoxide liquid bottoms is withdrawn and cooled by expansion; at least a portion of the expanded stream is used to cool the nitrogen-enriched overheads by indirect heat exchange; and a portion of the expanded stream is compressed and recycled to the column sump to provide reboil to the column.
In a first illustrated embodiment (FIG. 1) of U.S. Pat. No. 4,478,621, synthesis gas feed is partially condensed and the resultant two phase mixture fed to a wash column in which carbon monoxide is scrubbed from the vapor phase by contact with a liquid methane stream to provide CO-loaded methane containing some, typically 3-4%, hydrogen. A CO recycle heat pump stream provides intermediate indirect cooling to the wash column to remove the heat of solution of carbon monoxide in methane. Residual hydrogen is removed from the CO-loaded methane in a stripping column to meet the required carbon monoxide product specification. The hydrogen-stripped CO-loaded methane is separated into nitrogen-contaminated carbon monoxide overheads vapor and methane-rich bottoms liquid in a methane-separation fractionation column in which both overhears cooling and bottoms reboil is indirectly provided by the CO recycle heat pump stream. Nitrogen is removed from the carbon monoxide overheads in a nitrogen/CO fractionation column to provide CO product bottoms liquid. Overheads cooling to the nitrogen/CO fractionation column is indirectly provided by expanded CO product bottoms liquid and bottom reboil is directly provided by the CO recycle heat pump stream.
In a second illustrated embodiment (FIG. 2) of U.S. Pat. No. 4,478,621, synthesis gas feed of low methane content is fed into an open refrigerant cooling cycle derived from partial condensation of the resultant mixed feed. The mixed feed is compressed, partially condensed and then phase separated. The vapor phase from an initial phase separation is expanded to provide a hydrogen-rich product, a portion of which contributes to the refrigerant cooling cycle fluid. The liquid phase from the initial phase separation is degassed by expansion and subsequent phase separation. The expansion gas and a portion of the degassed liquid phase complete the refrigerant cooling cycle fluid. This fluid is warmed against process streams and added to the synthesis gas feed. The remainder of the degassed liquid is separated in a nitrogen/CO fractionation column to provide nitrogen-enriched overheads and CO product as bottoms liquid. As in the first embodiment, overheads cooling to the nitrogen/CO fractionation column is indirectly provided by expanded CO product bottoms liquid and bottom reboil is directly provided by a CO recycle heat pump stream.
The nitrogen-freed carbon monoxide liquid bottoms expansion usually has a pressure drop of only 1 to 2 Bar (100 to 200 kPa; 15 to 30 psi) because the stream is required to be subsequently warmed to ambient temperature to provide feed to the recycle compressor, which compressor feed should be slightly higher than atmospheric pressure. It is possible to use a compressor with a suction pressure below atmospheric pressure to increase the available pressure difference but this would significantly increase construction costs because of the need to provide protection for the compressor. The pressure difference available during expansion determines the extent of elevation of the liquid carbon monoxide level without the use of a pump. The difference can amount, for example, to about 12 metres at an expansion of 1 bar (100 kPa; 15 psi). If no additional pump is used, the maximum height of the distillation column is predetermined for each individual application.
It has been found that the limitation on the height of the distillation column without the use of a pump can be avoided by use, as a recycle heat pump stream, of a mixture of hydrogen and carbon monoxide obtained from the synthesis gas feed. This recycle stream can provide both reboiler and condenser duty to the nitrogen-separation column. The utilization of a single low pressure H.sub.2 /CO recycle system allows the nitrogen/carbon monoxide separation to be efficiently integrated into synthesis gas separation. Compared with the second embodiment of U.S. Pat. No. 4,478,621, the number of compressors required can be reduced (from two to one) because the presence of two separate heat pump cycles can be avoided.
GB-A-2297825 discloses the removal of nitrogen from a natural gas feed stream by a cryogenic distillation process in which the feed stream is separated in a distillation column to provide methane-rich bottoms liquid; nitrogen-rich overheads vapor and an intermediate vapor stream. The methane-rich bottoms liquid is recovered as a methane-rich product, preferably after being pumped to increase its pressure. The nitrogen-rich overhead vapor is warmed in heat exchange with the intermediate vapor stream to at least partially condense said stream for return to the distillation column to provide reflux. A portion of the warmed nitrogen-rich overhead vapor is utilized as a recycle nitrogen-rich heat pump stream above the critical pressure of nitrogen to provide at least part of the reboil to the distillation column and to produce a mixed vapor-liquid stream, which is returned to the distillation column to provide reflux.